AO node black artifacts
Nate suggested that you change the Auto Smooth value but in your file, this value panel is greyed out. This is because you have Custom Split Normals on your mesh.



Hit the Clear Custom Split Normals Data button and then try reducing the Auto Smooth Angle to something like 12 to 15 and that hopefully will clear your AO artifacts problem.



Edited to propose a different "solution"

I have no idea why but when moving the mesh and its origin to the World Origin a lot of the problem disappears! Perhaps someone can explain why?


After separating (P) a small part of the original model (less taxing on my PC) and moving it along with its origin to the World Origin most of the shading (shadowing) issues just disappeared:


Next some Vertex Smoothing to smooth out some of the sharper edges:


And finally with the auto smooth option unchecked: